A Comparative Study Of Normal Concrete And Recycled Concrete
CHAPTER I
INTRODUCTION
Normal concrete
Recycled Concrete
1.1 General
Any construction activity requires several materials such as concrete, steel, brick, stone, glass, clay, mud, wood, and so on. However, the cement concrete remains the main construction material used in construction industries. For its suitability and adaptability with respect to the changing environment, the concrete must be such that it can conserve resources, protect the environment, economize and lead to proper utilization of energy. To achieve this, major emphasis must be laid on the use of wastes and byproducts in cement and concrete used for new constructions. The utilization of recycled aggregate is particularly very promising as 75 per cent of concrete is made of aggregates. In that case, the aggregates considered are slag, power plant wastes, recycled concrete, mining and quarrying wastes, waste glass, incinerator residue, red mud, burnt clay, sawdust, combustor ash and foundry sand. The enormous quantities of demolished concrete are available at various construction sites, which are now posing a serious problem of disposal in urban areas. This can easily be recycled as aggregate and used in concrete.
The recycling and reuse of construction & demolition wastes seems feasible solution in rehabilitation and new constructions after the natural disaster or demolition of old structures. This becomes very important especially for those countries where national and local policies are stringent for disposal of construction and demolition wastes with guidance, penalties, levies etc.
And attempt has been made to find out feasibility of using recycled concrete instead of reinforced concrete.
1.2 Objective of the study:
1) To find out feasibility of using recycled concrete instead of normal concrete.
2) To prepare a mix design ratio of 1:1.5:3 and test 3 sets of cylinders at 14,21,&28 days for compressive strength using fresh reinforce concrete.
3) To prepare three sets of cylinders of recycled concrete using the same mix design ratio and test the specimens at 14, 21, & 28 days for compressive strength.
4) To compare the compressive strength of normal concrete with recycled concrete at different days.
5) To compare the cost of recycled and fresh concrete.
6) To assess the effect of demolished concrete on environment and friendly use of recycled concrete.
7) To draw few recommendations based on the study.
1.3 Scope & Limitation of the study:
In this study concrete waste collected from breaking of pile head is used. Only one mix design ratio is used to prepare the cylinders. For time limitation and limited budget only six sets of cylinders were prepared for testing. For further study waste concrete collected from several demolished construction, different mix design ratio can be used.
Chapter ii
Literature review
2.1 Concrete
Concrete is an artificial stone manufactured from a mixture of binding materials and inert materials with water.
Concrete = Binding materials + Inert materials + Water.
Concrete is considered as a chemically combined mass where the inert material acts as a filler and the binding material acts as a binder. The most important binding material is cement and lime. The inert materials used in concrete are termed as aggregates. The aggregates are of two types namely,
(1) Fine aggregate and
(2) Course aggregate.
Concrete is a composite construction material, composed of cement (commonly Portland cement) and other cementitious materials such as fly ash and slag cement, aggregate (generally a coarse aggregate made of gravel or crushed rocks such as limestone, or granite, plus a fine aggregate such as sand), water and chemical admixtures.
The word concrete comes from the Latin word “concretes” (meaning compact or condensed), the perfect passive participle of “concrescere”, from “con-” (together) and “crescere” (to grow).
Concrete solidifies and hardens after mixing with water and placement due to a chemical process known as hydration. The water reacts with the cement, which bonds the other components together, eventually creating a robust stone-like material. Concrete is used to make pavements, pipe, architectural structures, foundations, motorways/roads, bridges/overpasses, parking structures, brick/block walls, footings for gates, fences and poles and even boats.
Concrete is used more than any other man-made material in the world. As of 2006, about 7.5 billion cubic metres of concrete are made each year-more than one cubic metre for every person on Earth.
2.2 Aggregates
Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel and crushed stone are used mainly for this purpose. Recycled aggregates (from construction, demolition and excavation waste) are increasingly used as partial replacements of natural aggregates, while a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted.
Decorative stones such as quartzite, small river stones or crushed glass are sometimes added to the surface of concrete for a decorative “exposed aggregate” finish, popular among landscape designers.
The presence of aggregate greatly increases the robustness of concrete above that of cement, which otherwise is a brittle material and thus concrete is a true composite material.
Redistribution of aggregates after compaction often creates in homogeneity due to the influence of vibration. This can lead to strength gradients.
2.2.1 Fine aggregate
Sand and Surki are commonly used as fine aggregate in Bangladesh. Stone screenings, burnt clays, cinders and fly-ash are sometimes used as a substitute for sand in making concrete. The fine aggregate should not be larger than 3/16 inch (4.75mm) in diameter.
2.2.2 Coarse aggregate
Brick khoa (broken bricks), broken stones, gravels, Pebbles, clinkers, cinders etc. of
(he size of 3/16 to 2 inch are commonly used as coarse aggregate in Bangladesh. It
may be remembered that 3/16 inch is the dividing line between fine and coarse
aggregates.
2.2.3 Functions of Aggregates in Concrete
The aggregate give volume to the concrete around the surface of which the binding material adheres in the form of a thin Him. In theory the voids in the coarse aggregate is filled up with line aggregate and again the voids in the Hue aggregate is filled up
With the binding materials. Finally, the binding materials as the name involve binds the individual units of aggregates into a solid mass with the help of water.
2.2.4 Qualities of Aggregates
Since at least three quarters of the volume of concrete is occupied by aggregate. It is not surprising that its quality is of considerable importance. Not only the aggregate limit the strength of the concrete, as weak aggregates can not produce a strong concrete, but also the properties of aggregates greatly affect the durability and structural performance of the concrete.
Aggregate was though, originally viewed as an inert material dispersed throughout the cement paste largely for economic reason, yet it is possible, however, to take an opposite view and to look on aggregate as a building material connected into a cohesive whole by means of cement paste, in a manner similar to masonry constructions. In fact aggregates are not truly inert and their physical, chemical and sometimes thermal properties influence the structural performance of a concrete.
Aggregates are cheaper than cement and it is therefore, economical to put into the mix as much as of the former and as little of the latter. But economy is not the only reason for using aggregate: it confers considerable technical advantage on concrete, which has a higher volume stability and better durability than the cement paste alone. The coarse aggregate should be clean, strong, durable and well grades and should be free from impurities and deleterious materials, such as salts, coal residue, etc.
2.3 Cement
Cement is a cementing or binding material used in engineering construction. It is manufactured from calcareous substance (compounds of calcium and magnesium) and is similar in many respects to the strongly hydraulic limes but possessing far greater hydraulic properties.
Portland cement is the most common type of cement in general usage. It is a basic ingredient of concrete, mortar and plaster. English masonry worker Joseph Aspdin patented Portland cement in 1824; it was named because of its similarity in color to Portland limestone, quarried from the English Isle of Portland and used extensively in London architecture. It consists of a mixture of oxides of calcium, silicon and aluminum. Portland cement and similar materials are made by heating limestone (a source of calcium) with clay and grinding this product (called clinker) with a source of sulfate (most commonly gypsum).
2.3.1 Chemical composition of cement
Followings are the chemical compositions of cement:
* Lime (CaO) = 60-67%
* Silica (SiO2) = 17-25%
* Alumina (A12O3) = 3-8%
* Magnesia (MgO) = 0.1 -4%
* Sulphur trioxide (SO3) = 1 -3%
* Iron Oxide (Fe2O3) = 0.5-6%
* Soda and Potash alkalis = 0.5-1%
2.3.2 Hardening
Process of gaining strength by the mass of cement concrete is known as hardening. Tri-Calcium Silicate (C3S) hydrated first and responsible for most of early strength of concrete. Strength acquired during first 7 days is mostly due to hydration of €38. Di-Calcium Silicate (€28) starts contributing strength after 7 days to a year.
2.3.3 Setting
Process of loosing plasticity is known as setting. Tri-Calcium Aluminates (CsA) responsible for early setting of cement. C3A does not contribute any strength.
Tetra Calcium Alumino Ferrite (C4AF) does not play any significant roll in setting and hardening properties. For delaying setting for 30 to 40 minutes add 1-3% gypsum powder in cement. Initial setting of cement, 45 min to 8-10 hrs. Final setting time, 5to 20 hrs. Progressive hardening time, 24 hrs to a year. Within 30 days 80-90% strength gain.
2.4 Water
Combining water with a cementitious material forms a cement paste by the process of hydration. The cement paste glues the aggregate together, fills voids within it and allows it to flow more freely.
Less water in the cement paste will yield a stronger, more durable concrete; more water will give a freer-flowing concrete with a higher <href=”#Workability” title=”Concrete”>slump. Impure water used to make concrete can cause problems when setting or in causing premature failure of the structure.
Hydration involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the cement hydration process gradually bond together the individual sand and gravel particles and other components of the concrete, to form a solid mass.
Reaction:
Cement chemist notation: C3S + H ? C-S-H + CH
Standard notation: Ca3SiO5 + H2O ? (CaO)·(SiO2)·(H2O)(gel) + Ca(OH)2
Balanced: 2Ca3SiO5 + 7H2O ? 3(CaO)·2(SiO2)·4(H2O)(gel) + 3Ca(OH)2
2.4.1 Functions of water in concrete
Water serves the following three purposes:
1. To wet the surface of aggregates to develop adhesion because the cement paste
adheres quickly and satisfactory to the wet surface of the aggregates than to a dry
surface,
2. To prepare a plastic mixture of the various ingredients and to impart workability to
concrete to facilitate placing in the desired position and
3. Water to also needed for the hydration of the cementing materials to set and harden
during the period of curing.
2.5 Advantage of concrete over other materials of construction
Followings are the advantage of concrete over other materials of construction:
• Concrete is free from defects and flaws which natural stones are associated,
• It can be manufactured to desired strength and durability with economy.
• It can be cast to any desired shape.
• Maintenance cost of concrete structures is almost negligible.
• Concrete does not deteriorate appreciably with age.
2.6 Workability of Concrete
The strength of concrete of given mix proportion is very seriously affected by the degree of its compaction ; it is therefore, vital that the consistency the mix be such that the concrete can be transported placed and finished sufficiently easily and without segregation. A concrete satisfying these condition is said to be workable but to say merely that workability determines the case of transportation, placement and finishing and the resistance of concrete to segregation is too loose a description of this vital property of concrete workability can be best defined as a physical property which is the amount of useful external and internal works necessary to produce of compaction of concrete.
Another term used to describe the state or fresh concrete is consistency. In a simple language, the word consistency refers to the firmness of a form of a substance or to the case with which it will flow. In case of concrete, consistency is sometimes taken to mean the degree of witness within limits. Wet concrete are more workable than dry concrete, concretes of the same consistency may vary in workability.
2.6.1 Factors affecting workability
The main factor is the water content of the mix, expressed in pounds per cube yard of concrete. It is convenient, though approximate, to assume that for a given type and grading of aggregates and workability of concrete. The water content is independent of the aggregate cement ratio. On the basis of this assumption the mix proportions of concretes of different richness can be estimated and the following Table 2.1 gives typical values of water content for different slumps and maximum size of the aggregates.
Workability is also governed by the maximum size of the aggregates their grading, shape and texture. Grading and water/cement ratio have to be considered together as a grading producing most workable concrete for one particular value of water/cement ratio may not be the best for another value of the ratio. In particular, the higher the water/cement ratio the finer the grading required for the highest workability. In actual fact, for a given value of water/cement ratio, there is only one value of the coarse/fine aggregates ratio that gives the highest workability.
Air entrainment also increases workability. In general terms, entrainment of 5 percent air increases the compacting factor of concrete by about 0.03 to 0.07 and slump by 1/2 to 2 inch but actual values vary with properties of the mix. Air entrainment is also effective in improving the workability of the rather harsh mixes made with light weight aggregates.
The reason for the improvement of workability by the entrained air is probably that air bubbles act as a fine aggregate of very low surface friction and considerable elasticity. It is also claimed that the air entrainment reduces both segregation and bleeding.
Table 2.1: Approximate Water Content for different Slumps and Maximum sizes of Aggregates
| Maximum size of
aggregates inch |
Water content in Ib per cu yd. of concrete | |||||
| 1-2 inch slump | 3-4 inch slump | 6-7 inch slump | ||||
| Rounded Agg. | Angular
Agg. |
Rounded
Agg. |
Angular
Agg. |
Rounded Agg. | Angular Agg. | |
| 3/8 | 320 | 360 | 340 | 380 | 390 | 430 |
| 3/4 | 290 | 330 | 320 | 350 | 350 | 380 |
| l!/2 | 270 | 290 | 290 | 320 | 320 | 350 |
| 2 | 250 | 280 | 280 | 300 | 300 | 330 |
| 3 | 230 | 260 | 260 | 280 | 270 | 310 |
2.6.2 Measurement of Workability
Unfortunately no test is known that will measure directly the workability, numerous attempts have been made, however, to correlate workability with some easily measurable parameter. But none of these is fully satisfactory although they may provide useful information within a range of variation in workability. Water content for different size of aggregates is followed as per Table 2.1 as shown above.
2.7 Factors controlling properties of Concrete
The properties (Strength, durability, impermeability and workability) of concrete depend upon the following parameters (factors):
1. Grading of the aggregates.
2. Moisture content of the aggregates.
3. Water/cement ratio.
4. Proportioning of the various ingredients of concrete.
5. Method of mixing.
6. Placing and compaction of concrete.
7. Curing of concrete.
2.7.1 Water/Cement ratio
In engineering practices, the strength of concrete at a given age and cured at a prescribed temperature is assumed to depend primarily of two factors:
1. The water/cement ratio and
2. The degree of compaction.
The proportion between the amount of water and cement used in a concrete mix is termed as the water cement ratio.
The water in the concrete does primarily the three functions:
1. To wet the surface of the aggregate,
2. To impart workability and
3. To combine chemically with cement.
When concrete is fully compacted, its strength is taken to be inversely proportional to water-cement ratio. It may be recalled that the water-cement ratio determines the porosity of the hardened cement paste at any stage of hydration.
Experiments have shown that the quality of water in a mix determines its strength and there is a water/cement ratio which gives the maximum strength to the concrete. It will be found that there is a certain percentage of water below which the water will not be sufficient to hydrate the cement. The use of less water than that required will not give workability and will produce porous and weak concrete. On the other hand if more water is used than that actually required, the concrete will be weak.
2.8 Concrete Recycling:
When structures made of concrete are demolished or renovated, concrete recycling is an increasingly common method of utilizing the rubble. Concrete was once routinely trucked to landfills for disposal, but recycling has a number of benefits that have made it a more attractive option in this age of greater environmental awareness, more environmental laws, and the desire to keep construction costs down.
Concrete aggregate collected from demolition sites is put through a crushing machine. Crushing facilities accept only uncontaminated concrete, which must be free of trash, wood, paper and other such materials. Metals such as rebar are accepted, since they can be removed with magnets and other sorting devices and melted down for recycling elsewhere.
2.9 Recycled concrete
Concrete is one of the most important construction materials. Approximately one ton of concrete used per capita per year throughout the world. This enormous dependence on concrete is a compelling economic justification to seek improvements and new applications for a material that has been, in more ways than one, the foundation of major construction works. (Lee and Shah, 1988.) According to Kriejger (1980), concrete recycling usually involves cement concrete pavements along roadways. Meanwhile, advances in the design and construction of concrete structures since World War II imply demolition and disposal problems for the concrete components in these structures when they eventually reach the end of their useful lives. The need to recycle concrete components may be more than just cost-oriented. Advantages of recycling concrete pavements include reduced costs from aggregate produced on the job; reduced disposal costs and environmental damage; and the conservation of natural resources, i.e. aggregate and energy. Moreover, valuable landfill space is not used up. Recycled coarse aggregates may be more durable than virgin materials because they have already gone through years of freeze-thaw cycles. Conservation of natural resources includes reductions in the use of petroleum-based products and Portland cement, in aggregate quarrying and in iron ore mining. The fact that asphalt concrete, Portland cement concrete and iron can be recycled completely without requiring disposal also indirectly contributes to efforts for preserving the environment. The fact that recycling concrete has proved advantageous in pavement and road building encourages its use in residential construction as well. In the Netherlands, so much work is being done regarding the recycling of C&D waste in order to produce aggregates that a framework for a certification system
2.10 Importance of recycling
Recycling is the process of changing old and used products into new products to reduce pollution and prevent the waste of useful material.
Without recycling, some important metals would be entirely used up in the next 50 to 100 years. For example, there would be no more zinc by 2037 without recycling.
Normal landfills, where regular trash goes, give off many toxic and dangerous chemicals. These include gases that contribute to acid rain. Especially important to the world today is the release of methane and carbon dioxide. Both are greenhouse gases and contribute to climate change.
It takes less energy to recycle old products than make new ones entirely from ‘scratch’. For instance, it takes little energy to recycle an aluminum can. However, it takes a lot of energy to produce entirely new aluminum cans as it is expensive to extract aluminum from its ore.
It creates jobs and stimulates the economy. People have to drive trucks to pick up the recycling and the recycling plants employ lots of workers.
2.11. Uses of recycled concrete
Smaller pieces of concrete are used as gravel for new construction projects. Sub-base gravel is laid down as the lowest layer in a road, with fresh concrete or asphalt poured over it. The Federal Highway Administration may use techniques such as these to build new highways from the materials from old highways. Crushed recycled concrete can also be used as the dry aggregate for brand new concrete if it is free of contaminants. Larger pieces of crushed concrete, such as riprap, can be used for erosion control With proper quality control at the crushing facility, well graded and aesthetically pleasing materials can be provided as a substitute for landscaping stone or mulch. Wire gabions (cages), can be filled with crushed concrete and stacked together to provide economical retaining walls. Stacked gabions are also used to build privacy screen walls (in lieu of fencing)
2.12 Benefits of Recycling concrete
There are a variety of benefits in recycling concrete rather than dumping it or burying it in a landfill.
Keeping concrete debris out of landfills saves landfill space.
Using recycled material as gravel reduces the need for gravel mining.
Using recycled concrete as the base material for roadways reduces the pollution involved in trucking material.
Recycling Concrete is becoming an increasingly popular way to utilize aggregate left behind when structures or roadways are demolished. In the past, this rubble was disposed of in landfills, but with more attention being paid to environmental concerns, concrete recycling allows reuse of the rubbl while also keeping construction costs down.
2.13 Uses of recycled concrete aggregate
1. Using recycled concrete is an accepted source of aggregate into new concrete by ASTM and AASHTO.
2. It is of high quality and meeting or exceeding all applicable state and federal specifications.
3. Recycled aggregates are lighter weight per unit of volume, which means less weight per cubic yard, resulting in reduced material costs, haul costs, and overall project costs.
4. It is currently being used in concrete and asphalt products with better performance over comparable virgin aggregates.
5. It means minimization of environmental impacts in an Urban Quarry setting.
6. It Offers a way to reduce landfill waste streams.
7. It weighs ten to fifteen percent (10%-15%) less than comparable virgin quarry products (concrete).
8. It provides for superior compaction and constructability.
2.14. Recycled and Reuse of Construction & Demolition Wastes in Concrete:
The recycling and reuse of construction & demolition wastes seems feasible solution in rehabilitation and new constructions after the natural disaster or demolition of old structures. This becomes very important especially for those countries where national and local policies are stringent for disposal of construction and demolition wastes with guidance, penalties, levies etc.
2.14.1. International Status
The extensive research on recycled concrete aggregate and recycled aggregate concrete (RAC) as started from year 1945 in various part of the world after second world war, but in a fragmented manner. First effort has been made by Nixon in 1977 who complied all the work on recycled aggregate carried out between 1945-1977 and prepared a state-of-the-art report on it for RILEM technical committee 37-DRC. Nixon concluded that a number of researchers have examined the basic properties of concrete in which the aggregate is the product of crushing another concrete, where other concentrated on old laboratory specimens. However, a comprehensive state-of-the-art document on the recycled aggregate concrete has been presented by Hansen & others in 1992 in which detailed analysis of data has been made, leading towards preparation of guidelines for production and utilization of recycled aggregate concrete.
It has been estimated that approximately 180 million tones of construction & demolition waste are produced each year in European Union. In general, in EU, 500 Kg of construction rubble and demolition waste correspond annually to each citizen. Indicatively 10% of used aggregates in UK are RCA, whereas 78,000 tons of RCA were used in Holland in 1994. The Netherland produces about 14million tons of buildings and demolition wastes per annum in which about 8 million tons are recycled mainly for unbound road base courses. The 285 million tons of per annum construction waste produced in Germany, out of which 77 million tons are demolition waste. Approximately 70% of it is recycled and reused in new construction work. It has been estimated that approximately 13 million tons of concrete is demolished in France every year whereas in Japan total quantity of concrete debris is in the tune of 10-15 million tons each year. The Hong Kong generates about 20 million tons demolition debris per year and facing serious problem for its disposal. USA is utilizing approximately 2.7 billion tons of aggregate annually out of which 30-40% are used in road works and balance in structural concrete work. The rapid development in research on the use of RCA for the production of new concrete has also led to the production of concrete of high strength/performance. Indian Status there is severe shortage of infrastructural facilities like houses, hospitals, roads etc. in India and large quantities of construction materials for creating these facilities are needed. The planning Commission allocated approximately 50% of capital outlay for infrastructure development in successive 10th & 11th five year plans. Rapid infrastructural development such highways, airports etc. and growing demand for housing has led to scarcity & rise in cost of construction materials. Most of waste materials produced by demolished structures disposed off by dumping them as land fill. Dumping of wastes on land is causing shortage of dumping place in urban areas. Therefore, it is necessary to start recycling and re-use of demolition concrete waste to save environment, cost and energy. Central Pollution Control Board has estimated current quantum of solid waste generation in India to the tune of 48 million tons per annum out of which, waste from construction industry only accounts for more than 25%. Management of such high quantum of waste puts enormous pressure on solid waste management system. In view of significant role of recycled construction material and technology in the development of urban infrastructure, TIFAC has conducted a techno-market survey on ‘Utilization of Waste from Construction Industry’ targeting housing /building and road segment. The total quantum of waste from construction industry is estimated to be 12 to 14.7 million tons per annum out of which 7-8 million tons are concrete and brick waste. According to findings of survey, 70% of the respondent have given the reason for not adopting recycling of waste from Construction Industry is “Not aware of the recycling techniques” while remaining 30% have indicated that they are not even aware of recycling possibilities. Further, the user agencies/ industries pointed out that presently, the BIS and other codal provisions do not provide the specifications for use of recycled product in the construction activities.
In view of above, there is urgent need to take following measures:- Sensitization/ dissemination/ capacity building towards utilization of construction & demolition waste. Preparation and implementation of techno-legal regime including legislations, guidance, penalties etc. for disposal of building & construction waste. Delineation of dumping areas for pre-selection, treatment, transport of RCA. National level support on research studies on RCA. Preparation of techno-financial regime, financial support for introducing RCA in construction including assistance in transportation, establishing recycling plant etc. Preparation of data base on utilization of RCA. Formulation of guidelines, specifications and codal provisions. Preparation of list of experts available in this field who can provide knowhow and technology on totality basis. Incentives on using recycled aggregate concrete-subsidy or tax exemptions. Realizing the future & national importance of recycled aggregate concrete in construction, SERC, Ghaziabad had taken up a pilot R&D project on Recycling and Reuse of Demolition and Construction Wastes in Concrete for Low Rise and Low Cost Buildings in mid nineties with the aim of developing techniques/ methodologies for use recycled aggregate concrete in construction. The experimental investigations were carried out in Mat Science laboratory and Institutes around Delhi/GBD to evaluate the mechanical properties and durability parameters of recycled aggregate concrete made with recycled coarse aggregate collected from different sources. Also, the suitability in construction of buildings has been studied. The properties of RAC has been established and demonstrated through several experimental and field projects successfully. It has been concluded that RCA can be readily used in construction of low rise buildings, concrete paving blocks & tiles, flooring, retaining walls, approach lanes, sewerage structures, sub base course of pavement, drainage layer in highways, dry lean concrete(DLC) etc. in Indian scenario. Use of RCA will further ensure the sustainable development of society with savings in natural resources, materials and energy. Experimental Investigations. In the present paper, an endeavor is made so as to compare some of the mechanical properties of recycled aggregate concrete (RAC) with the natural aggregate concrete (NAC). Since the enormous quantity of concrete is available for recycling from demolished concrete structures, field demolished concrete is used in the present study to produce the recycled aggregates. The concrete debris were collected from different (four) sources with the age ranging from 2 to 40 years old and broken into the pieces of approximately 80 mm size with the help of hammer & drilling machine. The foreign matters were sorted out from the pieces. Further, those pieces were crushed in a lab jaw crusher and mechanically sieved through sieve of 4.75 mm to remove the finer particles. The recycled coarse aggregates were washed to remove dirt, dust etc. and collected for use in concrete mix. The fine aggregate were separated out, and used for masonry mortar & lean concrete mixes, which is not part this reported study. But these were found to suit for normal brick masonry mortar and had normal setting and enough strength for masonry work. Concrete Mixes The two different mix proportions of characteristic strength of 20 N/ mm2 (M 20) and 25 N/mm2 (M 25) commonly used in construction of low rise buildings are obtained as per IS 10262 – 1982 or both recycled aggregate concrete and natural aggregate concrete. Due to the higher water absorption capacity of RCA as compared to natural aggregate, both the aggregates are maintained at saturated surface dry (SSD) conditions before mixing operations. The proportions of the ingredients constituting the concrete mixes are 1:1.5:2.9 and 1:1.2:2.4 with water cement ratio 0.50 & 0.45 respectively for M-20 & M-25 grade concrete. The ordinary Portland cement of 43 grade and natural fine aggregates (Haldane sand) are used throughout the casting work. The maximum size of coarse aggregate used was 20 mm in both recycled and natural aggregate concrete. The total two mixes were cast using natural aggregate and eight mixes were cast using four type of recycled aggregate concrete for M-20 & M-25. The development of compressive strength is monitored by testing the 150-mm cubes at 1, 3, 7, 14, 28, 56 and 90 days. In one set 39 cubes were cast for each mix. The cylinder strength and corresponding strain & modulus of elasticity were measured in standard cylinder of 150×300 mm size at the age of 28 days. The prism of size 150x150x700 mm and cylinder of size 150x300mm were cast from the same batches to measure Flexural strength and splitting tensile strength respectively. This paper reports the results of experimental investigations on recycled aggregate concrete. Properties of Recycled Concrete Aggregate Particle Size Distribution The result of sieve analysis carried out as per IS 2386 for different types of crushed recycled concrete aggregate and natural aggregates. It is found that recycled coarse aggregate are reduced to various sizes during the process of crushing and sieving (by a sieve of 4.75mm), which gives best particle size distribution. The amount of fine particles (<4.75mm) after recycling of demolished were in the order of 5-20% depending upon the original grade of demolished concrete. The best quality natural aggregate can obtained by primary, secondary & tertiary crushing whereas the same can be obtained after primary & secondary crushing incase of recycled aggregate. The single crushing process is also effective in the case of recycled aggregate. The particle shape analysis of recycled aggregate indicates similar particle shape of natural aggregate obtained from crushed rock. The recycled aggregate generally meets all the standard requirements of aggregate used in concrete. Specific Gravity and Water Absorption The specific gravity (saturated surface dry condition) of recycled concrete aggregate was found from 2.35 to 2.58 which are lower as compared to natural aggregates. Since the RCA from demolished concrete consist of crushed stone aggregate with old mortar adhering to it, the water absorption ranges from 3.05% to 7.40%, which is relatively higher than that of the natural aggregates. The Table 4 gives the details of properties of RCA & natural aggregates. In general, as the water absorption characteristics of recycled aggregates are higher, it is advisable to maintain saturated surface dry (SSD) conditions of aggregate before start of the mixing operations. Bulk Density the ridded & loose bulk density of recycled aggregate is lower than that of natural aggregate except recycled aggregate-RCA4, which is obtained from demolished newly constructed culvert. Recycled aggregate had passed through the sieve of 4.75mm due to which voids increased in ridded condition. The lower value of loose bulk density of recycled aggregate may be attributed to its higher porosity than that of natural aggregate. Crushing and Impact Values The recycled aggregate is relatively weaker than the natural aggregate against mechanical actions. As per IS 2386, the crushing and impact values for concrete wearing surfaces should not exceed 45% and 50% respectively. The crushing & impact values of recycled aggregate satisfy the BIS specifications except RCA2 type of recycled aggregate for impact value as originally it is low grade rubbles. Compressive Strength the average compressive strengths cubes cast are determined as per IS 516 using As expected, the compressive strength of RAC is lower than the conventional concrete made from similar mix proportions. The reduction in strength of RAC as compare to NAC is in order of 2- 14% and 7.5 to 16% for M-20 & M-25 concretes respectively. The amount of reduction in strength depends on parameters such as grade of demolished concrete, replacement ratio, w/c ratio, processing of recycled aggregate etc. Splitting Tensile & Flexural Strength. The average splitting tensile and flexural of recycled aggregate are determined at the age 1, 3, 7, 14, & 28 days varies from 0.30 -3.1 MPa and 0.95- 7.2 MPa respectively. The reduction in splitting and flexural strength of RAC as compared to NAC is in order of 5-12% and 4 -15% respectively. Modulus of Elasticity The static modulus of elasticity of RAC has been reported in Table 4 and found lower than the AC. The reduction is up to 15% .The reason for the lower static modulus of elasticity of RCA is higher proportion of hardened cement paste. It is well establish that Ec depends on Ec value of coarse aggregate, w/c ratio & cement paste etc. The modulus of elasticity is critical parameter for designing the structures, hence more studies are needed.
2.15. Durability
The following parameters were studied to assess the influence of recycled aggregates on durability of concrete.
Carbonation Freeze-Thaw Resistance CarbonationCO2 from the air penetrates
into the concrete by diffusion process. The pores (pore size>100nm) in the concrete in which this transport process can take place are therefore particularly crucial for the rate of carbonation. The carbonation tests were carried out for 90 days on the specimens (150x150x150mm) of recycled aggregate concrete and natural aggregate concrete in carbonation chamber with relative humidity of 70% and 20% CO2 concentration. The carbonation depths of recycled aggregate concretes for different grade were found from 11.5 to 14mm as compared to 11mm depth for natural aggregate concrete. This increase in the carbonation depth of RAC as compared to NAC, attributed to porous recycled aggregate due to presence of old mortar attached to the crushed stone aggregate.
2.16. Freeze-Thaw Resistance
In the freeze-thaw resistance test (cube method), loss of mass of the concrete made with recycled aggregate was found sometimes above and below than that of concrete made with natural aggregate. The results were so close that no difference in freeze thaw resistance (after 100 cycles) could be found. The literature also found that the effect of cement mortar adhering to the original aggregate in RAC may not adversely affect the properties of RAC.
2.17. Conclusion
Recycling and reuse of building wastes have been found to be an appropriate solution to the problems of dumping hundred of thousands tons of debris accompanied with shortage of natural aggregates. The use of recycled aggregates in concrete prove to be a valuable building materials in technical, environment and economical respect Recycled aggregate posses relatively lower bulk density, crushing and impact values and higher water absorption as compared to natural aggregate. The compressive strength of recycled aggregate concrete is relatively lower up to 15% than natural aggregate concrete. The variation also depends on the original concrete from which the aggregates have been obtained. The durability parameters studied at SERC(G) confirms suitability of RCA & RAC in making durable concrete structures of selected types. There are several reliable applications for using recycled coarse aggregate in construction. However, more research and initiation of pilot project for application of RCA is needed for modifying our design codes, specifications and procedure for use of recycled aggregate concrete.
CHAPTER III
TESTING AND ANALYSIS OF MATERIALS
3.1 General
To select the appropriate materials for concrete to get better strength as well as workability following laboratory test has been recommended by ASTM.
• Gradation of Coarse and fine aggregates
• Aggregate crushing value (ACV) of coarse aggregate
• Flakiness Index of coarse aggregate
• Unit weight of coarse and fine aggregate
• Specific gravity and water absorption of coarse and fine aggregate
• Fineness modulus of fine aggregate
3.2 Testing of wet concrete
To confirm the workability and expected strength following test for wet concrete is widely used at construction site
• Workability test.
• Slump test.
• Compaction factor..
• Spreading table.
• Two point test.
• Air content
• Setting time.
• Density
• Yield
3.3 Laboratory Test
Prior to commencement of Concrete mix design we should know the physical properties of materials. To know the physical properties of the materials following laboratory tests has been conducted in the laboratory.
3.3.1 Grading of Coarse aggregate
To get the better workability as well as strength the coarse aggregate should confirm with the specified limit of ASTM, which shown in Table 3.1 & 3.2. If we follow the proper grading the proportion of paste and aggregate will be good combination which will give us better strength and workability.
Table 3.1: Grading of Coarse aggregate
| Sieve size
(mm) |
Individu al Wt. Retained (gm) | Cumulative Wt. Retained (gm) | Cumulative % retained | Cumulative % Passing | Specified Limit
(% Passing) |
| 25 | 0 | 0 | 0 | 100 | 100 |
| 20 | 118 | 118 | 2.58 | 97.42 | 90-100 |
| 12.5 | 3093 | 67.71 | 67.71 | 32.29 | 20-55 |
| 10 | 883 | 3976 | 87.13 | 12.87 | 5-20 |
| 5 (#4) | 487 | 4463 | 97.80 | 2.87 | 0-5 |
| 0.075 (#200) | 4522 | 4524 | 99.10 | 0.90 | 0-1.5 |
| Pan | 41 | – | – | – | – |
| Total | 4563 | – | – | – | – |
Remarks: The cumulative percent passing is within the range of the specified limit. So, the test result is considered satisfactory.
Table 3.2: Grading of Coarse aggregate : (Recycled Concrete)
| Sieve size (mm) | Individual Wt. Retained (gm) | Cumulative Wt. Retained
(gm) |
Cumulative % retained | Cumulative % Passing | Specified Limit (% Passing) |
| 25 | 200 | 200 | 4.86 | 95.1 | 90-100 |
| 20 | 310 | 510 | 14.71 | 85.29 | 90-100 |
| 12.5 | 2920 | 3630 | 98.9 | 1.1 | 5-20 |
| Pan | 38 | – | – | – | – |
| Total | 3468 | – | – | – | – |
Remarks: The cumulative percent passing is within the range of the specified limit. & some results are very near to the specified limit. So the test result is considered satisfactory.
3.3.2 Aggregate crushing value (ACV)
The aggregate crushing value gives a relative measure of the resistance of an aggregate to crushing under a gradually applied compressive load. With aggregate of an aggregate crushing value higher than 30, the result may be anomalous. The standard aggregate crushing test shall consist of aggregate passing the 14mm B.S Test Sieve and retained on the 10mm B.S Test Sieve. The specified value of ACV of ASTM shown in Table 3.3 & 3.4
Table 3.3: Aggregate Crushing value (ACV) Test (Stone chips)
| Test No. | 1 | 2 | Specified Limit |
| Weight of surface dry Aggregate before test, A, gm. ( Aggregate Passing a 14.00 mm Sieve and Retained on a 10.00 mm Sieve) | 2875 | 2860 | Less than 30% |
| Weight of Aggregate Passing through 2.36 mm
(# 8) Sieve after test. B gm. B gm. |
722 | 706 | |
| Maximum load ( at 10 minutes duration ) KN | 400 | 400 | |
| B
Aggregate Crushing value, ACV = — x100 % A |
25.11 | 24.69 | |
| Average (Mean) Aggregate Crushing Value, (ACV), % | 25.00 | ||
Remarks: The value for ACV test can not be greater than 30%, we get 25 %, which is less than the specified limit. So, the result is satisfactory.
Table 3.4: Aggregate Crushing value (ACV) Test (Recycled concrete)
| Test No. | 1 | 2 | Specified Limit |
| Weight of surface dry Aggregate before test, A,gm. ( Aggregate Passing a 14.00 mm Sieve and Retained on a 10.00 mm Sieve) | 3050 | 3170 | L
L Less than 30% |
| Weight of Aggregate Passing through 2.36 mm (# 8) Sieve after test. B gm | 880 | 920 | Less than 30%
30% |
| Maximum load ( at 1 0 minutes duration ) KN | 400 | 400 | 30% |
| Aggregate Crushing value, ACV = ( B/A) * 1 00% | 28.85 | 29.02 | |
| Average (Mean) Aggregate Crushing Value, (ACV),% | 28.93 | ||
Remarks: The value for ACV test can not be greater than 30%, we 28.93 %, which is less than the specified limit. So, the result is satisfactory.
3.3.3 Flakiness Index
This test is based on the classification of aggregate particles as flaky when they have a thickness (smallest dimension) of less than 0.6 of their nominal size, this size being taken as the mean of the limiting sieve apertures used for determining the sieve fraction in which the particle occurs. The flakiness index often aggregate sample is found by separating flaky particles and expressing their mass as a percentage of the mass of the sample tested. The test is not applicable to material passing a 6.3mm B.S Test Sieve and retains on a 63mm B.S Test Sieve. The flakiness index test as shown in Table 3.5.
Table 3.5: Flakiness Index (F.I) Test (Stone chips)
| Aggregate Size | Tested weight (gm) | Wt. Retained
(gm) |
Wt.
Passing (gm) |
% Flaky (Individual) | Specified Limit |
| 50 mm -37.5 mm | – | – | – | – | Less than 30% |
| 37.5 mm -28 mm | – | – | – | – | |
| 28 mm -20 mm | 100 | 100 | 0 | 0 | |
| 20 mm – 14 mm | 2080 | 1705 | 375 | 18.03 | |
| 14 mm- 10 mm | 1535 | 1225 | 310 | 20.20 | |
| 10 mm -6.3 mm | 1285 | 1002 | 283 | 22.02 | |
| 6.3 mm Not tested | 210 | ||||
| Total Wt. ( gm ) | 5210 | 968 |
Calculation: Flakiness Index =
| Total Wt. passing through gauges x 100%
Total Wt. of Test Sample |
F.I = (968/5210) x 100%
F.I = 18.58 %
Remarks: The percentage of flaky should not exceed the range of 30%. We get both individual and total Percentage of flaky less than 30%. So, the result is satisfactory.
Table 3.6: Flakiness Index (F.I) Test (Recycled concrete)
| Aggregate Size | Tested weight (gm) | Wt. Retained
(gm) |
Wt.
Passing (gm) |
% Flaky (Individual) | Specified Limit |
| 50 mm -37.5 mm | – | – | – | – | Less than 30% |
| 37.5 mm -28 mm | 620 | 510 | 110 | 17.74 | |
| 28 mm -20 mm | 2715 | 2118 | 597 | 21.98 | |
| 20 mm – 14 mm | 1080 | 880 | 200 | 18.51 | |
| 14 mm- 10 mm | 615 | 135 | 480 | 78.04 | |
| 10 mm -6.3 mm | – | – | – | – | |
| 6.3 mm Not tested | 180 | ||||
| Total Wt. ( gm ) | 5210 | 1387 |
Calculation: Flakiness Index =
| Total Wt. passing through gauges x 100%
Total Wt. of Test Sample |
F.I = (1387/5210) x 100%
F.I = 26.26 %
Remarks: The percentage of flaky should not exceed the range of 30%. We get both individual and total Percentage of flaky less than 30%. So, the result is satisfactory.
3.3.4 Grading of fine aggregate
The grading of fine aggregate to be done to maintain the better property of aggregate in concrete mix . It has been seen that if the fine aggregate is in ASTM specified limit those have given better workability and strength. The specified limit of ASTM for grading of fine aggregate is shown in Table 3.6 & 3.7.</p